Method for controlling the transient response of a power converter powering a load, transient response controller and power converter

ABSTRACT

A method for controlling the transient response of a power converter powering a load, said power converter comprising a power switch, a synchronous rectifier and a capacitor coupled between an input and an output of the power converter, said method comprising the step of—disabling said synchronous rectifier in response to a signal indicative of a change of said load, is characterized by—providing said signal based on a current representing said change of load.

The invention relates to a method for controlling the transient responseof a power converter powering a load, said power converter comprising apower switch, a synchronous rectifier and a capacitor coupled between aninput and an output of the power converter, said method comprising thestep of disabling said synchronous rectifier in response to a signalindicative of a change of said load. The invention also relates to atransient response controller to perform the above method and to a powerconverter including such a transient response controller.

Power converters are subject to transient conditions, such as turn-onand turn-off transients, as well as sudden changes in load and inputvoltage. Future generations of high-speed digital integrated circuitssuch as high-performance processors, digital signal processors, systemon chip, etc., will operate at lower voltages with tighter tolerancesand increased dynamic load characteristics. These integrated circuitsare able to reduce their power consumption from maximum to minimumwithin a few nanoseconds. This time period is much too short for thepower supply to react. The supplied integrated circuit, after a turn-offtransient, requires only a small amount of current. Thus, the energystored in the buck coil charges the output capacitors, leading to ahigher supply voltage. Since tolerances in the supply voltage are verysmall, the capacitance at the output has to be chosen in order to limitvoltage excursion within this tolerance band. Consequently, manycapacitors are needed to fulfill the requirement, which is costintensive. Power converters therefore need new concepts.

Generally, a power converter comprises a power switch and a synchronousrectifier coupled between an input and an output of the power converter.Power switch and synchronous rectifier alternate between a conductiveand a non-conductive state. When the power switch conducts, thesynchronous rectifier is non-conductive and vice versa. A transientcondition occurs, as shown in FIG. 1, at a time instant t=0, when theload is removed. The output current suddenly drops to zero and theconverter output voltage rises above its nominal, steady-state value.The power switch is shut down and the synchronous rectifier remains in aconductive state. As a result, the converter output voltage rises to anundesirable level. Likewise, during this time, the output inductorcurrent I_(L) drops at a rate roughly proportional to the output voltagedivided by the inductance. The synchronous rectifier current drops atthe same rate.

Normally, the synchronous rectifier is embodied by a MOSFET, whichalways includes a back-gate diode or body diode. The power converterdisclosed in U.S. Pat. No. 5,940,287 A controls the synchronousrectifier by sensing that the power switch has been in a non-conductivestate for a given time period and after lapse of that time periodshutting down the synchronous rectifier, thus forcing conduction of thesynchronous rectifier's body diode and thereby limiting the converteroutput voltage. Due to the increased voltage drop across the body diodepart of the energy previously stored in the buck coil is now dissipatedin the body diode, thus leaving less energy to be discharged into theoutput capacitor. Since the information about the change of load istaken from the gate signal of the MOSFET, an RC-time constant isinvolved, which is longer than one complete switching period. Therefore,while reduced, the voltage overshoot is still larger than necessary.

It is the object of the invention to provide a method for controllingthe transient response of a power converter as defined in theintroduction which can minimize output voltage overshoot reliably andvery quickly.

In a first aspect of the invention, the object is solved in a method asdefined above by providing said current-based signal representing saidchange of load to cause said transient response controller toimmediately disable said synchronous rectifier without any time delay.This implementation is based on the principle of detecting a voltagerise across the capacitor and to counteract thereto by a suitablecorrecting measure. In a turn-off case, this correcting measure consistsin shutting off not only the power switch but also the synchronousrectifier, so that the buck coil current is dissipated through the bodydiode effecting the desired additional voltage drop, as is alreadydisclosed in U.S. Pat. No. 5,940,287. However, as seen from U.S. Pat.No. 5,940,287, a quick and accurate detection of a voltage change isless practicable due to the distractions to be expected and, therefore,one must wait until a measurable voltage rise occurs through chargingthe capacitor the voltage rise of which is furthermore indirectlyexploited by waiting for a non-occurrence of the switching signal forthe power switch. In contrast, the invention makes use of a currentmeasurement, either directly or indirectly, so that a measure tocounteract a decrease in the load can be initiated as early as possible.

Said current-based signal can be directly provided by said load. Forexample, when an integrated circuit or a microprocessor changes from anactive into a passive state, it can itself communicate this informationabout a change of power consumption and therefore needed load current tothe transient response controller which will then immediately shut offthe synchronous rectifier's MOSFET. Generally, a controller associatedwith the load will know in advance about power consumption and thereforecurrents within the load, so that shut-off periods of the synchronousrectifier can be finely tuned and adapted not only to changes from theoperational mode to the standby mode and vice versa, but also tospecific operations occurring during the operational mode. Thefine-tuning process can be implemented by comparing the current throughthe load or the current to be expected through the load with at leastone threshold value and using this information to derive the shut offperiods.

Another aspect of the invention uses the possibility to measure thecurrent I_(o) through the load, which, however, is not easy to achievedue to the physical implementation of the microprocessor supply.Basically, in the case of a turn-off transient, a decrease of thecurrent through the load must be detected. Since rapid recognition isrequired, a current through the buck coil may be regarded as constant.Therefore, the following approximation is correct: $\begin{matrix}{{\frac{\mathbb{d}I_{c}}{\mathbb{d}t} = {- \frac{\mathbb{d}I_{o}}{\mathbb{d}t}}},} & (1)\end{matrix}$

wherein I_(c) is the current through the output capacitor whichconsequently can be used equally. The output capacitor regularly is nota single element, but consists of a plurality of parallel-connectedcapacitors which are each characterized by parasitic serial resistanceR_(C) and serial inductance L_(C). The time constant L_(C)/R_(C)however, is independent of the number of capacitors and is in the rangeof hundreds of nanoseconds. Now, the voltage of one of these capacitorscan be measured. If this measured voltage is filtered by a first R₁C₁element satisfying $\begin{matrix}{{C_{1}R_{1}} = \frac{L_{C}}{R_{C}}} & (2)\end{matrix}$wherein

R_(C)=parasitic serial resistance of capacitor element

L_(C)=parasitic serial inductance of capacitor element

C₁=capacitance of first RC element

R₁=resistance of first RC element,

a signal is obtained comprising a portion which is nearly constant forthe time of the load transient, namely the voltage drop across the idealcapacitor C, and a portion proportional to the current, i.e. the voltagedrop across serial resistance R_(C). The condition in (2) compensatesfor the voltage drop across serial inductance L_(C).

A preferred embodiment undercompensates the voltage drop across theserial inductance L_(C) by requiring $\begin{matrix}{{C_{1}R_{1}} < \frac{L_{C}}{R_{C}}} & (3)\end{matrix}$

thus emphasizing a portion proportional to the change of current.

It is the advantage of the previous embodiments that said first filterstage shows low pass characteristics which is favorable with respect tointerference susceptibility.

As before, the method can be finely tuned by comparing the current orsignal with at least one threshold value.

The object is also solved by a transient response controller to be usedin a power converter powering a load, said power converter comprising apower switch, a synchronous rectifier and a capacitor coupled between aninput and an output thereof, said transient response controller beingcoupled at least to said synchronous rectifier and disabling saidsynchronous rectifier in response to a signal indicative of a change ofsaid load, characterized in that said transient response controller iscoupled to means for providing said signal based on a currentrepresenting said change of load.

Finally, the object is solved by a power converter powering a load whichincludes the transient response controller defined above. Said means forproviding said signal comprises means for detecting the current throughsaid load or means for detecting the voltage drop across said capacitoras well as means for comparing said current or voltage drop with atleast one threshold value.

It is preferable that said means for providing said signal is acontroller of said load communicating the power consumption of said loadto said transient response controller.

Such a power converter can be used for powering high speed integratedcircuits.

In the following, the invention will be described in further detail withreference to the accompanying drawing, wherein

FIG. 1 illustrates timing diagrams for a power converter withouttransient response control during a turn off transient;

FIG. 2 illustrates a schematic diagram of a half bridge of a powerconverter;

FIG. 3 illustrates timing diagrams for a power converter of the priorart during a turn off transient;

FIG. 4 illustrates a schematic diagram of a power converter embodyingthe present invention;

FIG. 5 shows an equivalent circuit diagram of the output capacitor;

FIG. 6 illustrates a preferred embodiment of the method for controllingthe transient response of a power converter according to the invention;and

FIG. 7 illustrates timing diagrams for a synchronous rectifiercontrolling scheme according to the invention.

Referring initially to FIG. 1, exemplary current waveforms areillustrated for a power converter which has no special transientsynchronous rectifier control. The wave form referenced by I_(o)represents the converter output current I_(o), “M” represents the stateof the power switch when in either a conducting or non-conducting state,“SR” represents the state of the synchronous rectifier when it is ineither a conducting or non-conducting state, and “IL” represents theoutput inductor current I_(L) over the time period observed. As can beseen from FIG. 1, the power switch and synchronous rectifier alternatebetween a conductive and a non-conductive state such that when the powerswitch conducts the synchronous rectifier is non-conductive and viceversa. During normal operation, the converter output voltage and thecurrent through the output inductor remain constant within certainlimitations. When the output current I_(o) suddenly drops to zero, thenormal power converter cannot reduce the value I_(L) quickly enough. Thecharge represented by the black area charges the output capacitorleading to a voltage overshoot.

FIG. 2 is a schematic diagram of a half bridge which is provided toillustrate the controlling scheme of the prior art. A power switch T1and a synchronous rectifier T2 are both embodied by a MOSFET, whereinthe gate G of each MOSFET is controlled by a respective driver D1 andD2. A buck coil B stores energy as described above. If an transientcondition is detected, the control scheme of U.S. Pat. No. 5,940,287shuts down not only the power switch T1, but also, after the powerswitch T1 has been in a non-conductive state for a given time period,synchronous rectifier T2. Bypassing the current to the intrinsic bodydiode BD of the MOSFET will dissipate part of the energy stored in thebuck coil.

According to the control scheme of U.S. Pat. No. 5,940,287 and asillustrated in FIG. 3, the controller has to wait at least until thenext timing signal would occur at the power switch before shutting downthe synchronous rectifier. Conduction through the body diode is thenforced which limits the converter output voltage V_(o).

FIG. 4 illustrates a schematic diagram of a power converter embodyingthe present invention. The power converter comprises half bridges 20 ₁to 20 _(n), each of them having a similar construction than those shownin FIG. 2 with their respective buck coil 22 ₁ to 22 _(n). Signals froma controller 24 are given to inputs D1 ₁, D2 ₁ to D1 _(n), D2 _(n) tocontrol the circuitry within half bridges 20 ₁ to 20 _(n). An outputcapacitor 30 consisting of parallel connected capacitor elements C₁, C₂to C_(N), is coupled to the output of the power converter. The converteroutput voltage V₀ is measured across capacitor 30. Further, a load 10 iscoupled across the output capacitor 30. Current I_(B) from the buckcoils 22 ₁ to 22 _(n) branches into current I_(o) through load 10 andcurrent I_(C) to capacitor 30. Boxes 42, 44 symbolize the detection ofeither current I_(o) or current I_(C).

A possible embodiment of the control scheme of the present inventionwill be explained with respect to changes of current I_(C) throughcapacitor 30, provided that in case of equal capacitances an equivalentcircuit diagram applies as shown in FIG. 5.

The resulting overshoot is lower since the charge indicated by the blackarea in FIG. 7 is lower than in the previous methods.

FIG. 6 shows that the voltage across the capacitor is tapped andfiltered by a first RC element with a resistance R₁ and capacitance C₁to satisfy equation (2) above. The resulting signal S2 includes acomponent proportional to the current I_(C), as explained above withrespect to equation (2). Optional, an impedance converter can beprovided to output signal S3 which is input into a high pass filter orsecond RC element, wherein capacitance C₂ and resistance R₂ thereof areselected to satisfyC₂R₂>>C₁R₁   (4)

to filter the constant component from the signal. The resulting signalS4 is then amplified and; amplified signal S5 is given into a comparatorwhich detects whether or not signal S5 exceeds a predetermined thresholdvalue. If a threshold value is exceeded, signal S6 is changed from lowto high. The high signal is then given to controller 24 to shut off ofboth power switch T1 and synchronous rectifier T2. In a further improvedembodiment the comparator will have two or more threshold values tosignal to controller 24 that a smaller or larger current rise isoccurring which leads to a respective smaller or larger voltage rise.Thus, controller 24 is enabled to set either synchronizing as usual orbypassing the body diode, as the case will be.

Additionally, threshold values representing negative currents can bepredetermined to effect the termination of the body diode conductionmode early enough to ensure that power converter operation is notdistracted.

FIG. 7 shows the timing diagram wherein drop of the current I_(L) setson immediately, so that voltage overshoot can be minimized. In “IL” ofFIG. 7, the comparison with the graphs of FIGS. 1 and 3 are shown indotted lines. As can be seen in the Figures, the effect is dramaticallypositive with decreasingly required supply voltage.

1. A method for controlling the transient response of a power converterpowering a load (10), said power converter comprising a power switch(T1), a synchronous rectifier (T2) and a capacitor (30; C₁, C₂, . . .C_(N)) coupled between an input and an output of the power converter,said method comprising the step of disabling said synchronous rectifier(T2) in response to a signal indicative of a change of said load (10),characterized by providing said signal based on a current representingsaid change of load.
 2. The method as claimed in claim 1, characterizedin that said load (10) communicates information about its needed currentto provide said signal.
 3. The method as claimed in claim 1,characterized in that said signal is provided by detecting a current(I_(o)) through said load (10).
 4. The method as claimed in claim 1,characterized in that said signal is provided by detecting a current(I_(C)).
 5. A method for detecting the transient response of a powerconverter powering a load (10), characterized by filtering a voltageacross said capacitor (30) by a first RC element, said first RC elementsatisfying ${C_{1}R_{1}} \leq \frac{L_{C}}{R_{C}}$ whereinR_(C)=parasitic serial resistance of capacitor L_(C)=parasitic serialinductance of capacitor R₁=resistance of first RC element C₁=capacitanceof first RC element
 6. The method as claimed in any of claims 1 to 5,characterized that said signal based on a current is compared to atleast one threshold value.
 7. Transient response controller to be usedin a power converter powering a load (10), said power convertercomprising a power switch (T1), a synchronous rectifier (T2) and acapacitor (30; C₁, C₂ . . . C_(N)) coupled between an input and anoutput thereof, said transient response controller being coupled atleast to said synchronous rectifier (T2) to disable said synchronousrectifier in response to a signal indicative of a change of said load(10), characterized in that said transient response controller (40) iscoupled to means for providing said signal based on a currentrepresenting the change of load.
 8. A power converter powering a load,comprising a power switch (T1), a synchronous rectifier (T2) and acapacitor (30; C₁, C₂ . . . C_(N)) coupled between an input and anoutput of the power converter, and a transient response controller (40)coupled to at least said synchronous rectifier T2, said transientresponse controller (40) disabling said synchronous rectifier inresponse to a signal indicative of a change of said load (10), by meansfor providing said signal based on a current representing said change ofload, said means for providing said signal being coupled to saidtransient response controller (40).
 9. The power converter as claimed inclaim 8, characterized in that said means for providing said signal is acontroller of said load (10) communicating the power consumption of saidload (10) to said transient response controller (40).
 10. The powerconverter as claimed in claim 8, characterized in that said means forproviding said signal comprises means for detecting the current throughsaid load (10) and means for comparing said current (I_(o)) with atleast one threshold value.
 11. The power converter as claimed in claim8, characterized in that said means for providing said signal comprisesmeans for detecting the current (I_(C)) through said capacitor (30) by avoltage drop across said capacitor (30) and means for comparing saidvoltage drop with at least one threshold value.
 12. The power converteras claimed in any of claims 8 to 11, characterized in that saidtransient response controller (40) is connected to said power switch(T1) to switch off said power switch in response to said signal.
 13. Useof power converter as claimed in any of claims 8 to 12 for powering highspeed integrated circuits.